1. Field of the Invention
The present invention relates to an improvement in a laser theodolite capable of emitting a laser beam in a direction of collimation.
2. Description of the Related Art
An optical structure such as that shown in FIG. 1 is known as a laser theodolite. An objective lens 50, a focusing lens 51, a reticle 52, and an eyepiece 53 are provided within the lens barrel of the telescope of the laser theodolite. Also, there is provided a half mirror 54 between the focusing lens 51 and the reticle 52. With this arrangement, a laser beam from a helium-neon laser light source 55 is reflected in a direction of collimation. Furthermore, a movable laser-beam focusing lens 56, a total reflecting mirror 57, and a movable laser-beam focusing lens 58 are provided between the helium-neon laser light source 55 and the half mirror 54. If the movable lenses 56 and 58 are moved in the optical axis direction, a laser beam from the helium-neon laser light source 55 is focused at position Q and a real image is formed. A lens 59 is provided between the movable lens 58 and the half mirror 54. The focal distance of the lens 59 is longer than the distance from the position of the lens 59 to the position Q. The virtual image Q' at the position Q which is formed by the lens 59 and the reticle 52 are conjugate with respect to the half mirror 54.
However, in this conventional laser theodolite, the half mirror 54 reflects part of the laser beam in a direction of collimation and transmits the remaining part from the essential nature of the half mirror 54. Also, the half mirror 54 reflects part of collimated light, which is incident from a collimation point existing in the direction of collimation toward the collimating system of the telescope, and at the same time, transmits the remaining part. Because of the existence of the half mirror 54, part of the collimated light is reflected and the visual field becomes dark, and consequently, this conventional laser theodolite has the disadvantage that the surveying operation is difficult to perform accurately and quickly. Also, when the laser beam from the helium-neon laser light source 55 is reflected in the direction of collimation by the half mirror 54, part of the laser beam is transmitted through the half mirror 54. For this reason, the light quantity of the laser beam which is reflected toward the collimation point by the half mirror 54 is reduced. Similarly, part of the reflected laser beam from the collimation point is reflected by the half mirror 54, and the reflected laser beam which is transmitted toward the reticle 52 is reduced in quantity. Consequently, there is the disadvantage that the reflected laser beam from the collimation point is also difficult to collimate. Furthermore, the laser beam, transmitted in the direction of collimation without being reflected by the half mirror 54, becomes a ghost image easily. For example, in the case where visual laser light with red color is employed, the conventional laser theodolite has the disadvantage that the entire visual field looks reddish.